1. Field of the Inventions
The present inventions generally relate to apparatus and methods of solar cell design and fabrication and, more particularly, to forming contact layers and diffusion barriers for such solar cells.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical energy. Solar cells can be based on crystalline silicon or thin films of various semiconductor materials, usually deposited on low-cost substrates, such as glass, plastic, or stainless steel.
Thin film based photovoltaic cells, such as amorphous silicon, cadmium telluride, copper indium diselenide or copper indium gallium diselenide based solar cells, offer improved cost by employing deposition techniques widely used in the thin film industry. Group IBIIIAVIA compound photovoltaic cells including copper indium gallium diselenide (CIGS) based solar cells have demonstrated the greatest potential for high performance, high efficiency, and low cost thin film PV products.
A conventional Group IBIIIAVIA compound solar cell can be built on a substrate that can be a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. A contact layer such as a molybdenum (Mo) film is deposited on the substrate as the back electrode of the solar cell. An absorber thin film including a material in the family of Cu(In,Ga)(S,Se)2, is formed on the conductive Mo film. Although there are other methods, Cu(In,Ga)(S,Se)2 type compound thin films are typically formed by a two-stage process where the components (components being Cu, In, Ga, Se and S) of the Cu(In,Ga)(S,Se)2 material are first deposited onto the substrate or the contact layer formed on the substrate as an absorber precursor, and then reacted with S and/or Se in a high temperature annealing process.
After the absorber film is formed, a transparent layer, for example, a CdS film, a ZnO film or a CdS/ZnO film-stack is formed on the absorber film. The preferred electrical type of the absorber film is p-type, and the preferred electrical type of the transparent layer is n-type. However, an n-type absorber and a p-type window layer can also be formed. The above described conventional device structure is called substrate-type structure. In the substrate structure light enters the device from the transparent layer side. A so called superstrate-type structure can also be formed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga)(S,Se)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In the superstrate structure light enters the device from the transparent superstrate side.
In standard CIGS as well as Si and amorphous Si module technologies, the solar cells can be manufactured on flexible conductive substrates such as stainless steel foil substrates. Due to its flexibility, a stainless steel substrate allows low cost roll-to-roll solar cell manufacturing techniques. In such solar cells built on conductive substrates, the transparent layer and the conductive substrate form the opposite poles of the solar cells. Multiple solar cells can be electrically interconnected by stringing or shingling methods that establish electrical connection between the opposite poles of the solar cells. Such interconnected solar cells are then packaged in protective packages to form solar modules or panels. Many modules can also be combined to form large solar panels. The solar modules are constructed using various packaging materials to mechanically support and protect the solar cells in them against mechanical damage. Each module typically includes multiple solar cells which are electrically connected to one another using above mentioned stringing or shingling interconnection methods.
The conversion efficiency of a thin film solar cell depends on many fundamental factors, such as the bandgap value and electronic and optical quality of the absorber layer, the quality of the window layer, the quality of the rectifying junction, and so on. Since the total thickness of the electrically active layers of the CIGS thin film solar cells is in the range of 0.5-5 micrometers, these devices are highly sensitive to defects. Even the sub-micron size defects may influence their illuminated I-V characteristics. A prior art problem associated with manufacturing CIGS thin film devices on stainless steel substrates, however, is the inadvertent introduction of defects into the device structure during the reaction of the absorber precursor. During this step, the constituents of the stainless steel substrate diffuse towards the absorber layer, and Se from the absorber precursor diffuses towards the stainless steel substrate. Despite the advantages of flexible stainless steel substrates in CIGS solar cell applications, during the high temperature anneal, Se from the absorber precursor can diffuse through the Mo contact layer toward the substrate, corrode the stainless steel and form FeSe compounds on the substrate, which cause electrical shunting defects. Such shunting defects introduce a shunting path, between the substrate and the absorber or the transparent layer through which the electrical current of the device may leak. The shunting defects lower the fill factor, the voltage and the conversion efficiency of the solar cells. Furthermore, the atoms of the stainless steel such as iron (Fe) atoms can diffuse from the substrate into the CIGS absorber by diffusing through the Mo contact layer during the aforementioned high temperature anneal or reaction process that forms the absorber. This unwanted material diffusion into the absorber significantly degrades the performance of the absorber and the resulting device. One prior art process uses a chromium (Cr) layer to prevent Fe diffusion into the absorber; however, Cr forms CrSe with Se from the absorber precursor during the reaction step, and just like FeSe, Cr causes shunting defects.